Earlier quoted context omitted.
Firefox has supported multiple profiles for ages. Just run it with -P. It allows you to have separate history, extensions, bookmarks, everything. The new panel containers are also handy. They allow you to have shared history and bookmarks but separate cookies and sessions. You can be logged into the same site on different accounts in two tabs in the same window.
There's no easy way of switching between them, starting a new firefox instance requires running from a command interpreter with -P specified, even the selection window isn't great. The containers feature is the alternative you want to propose here.
http://kb.mozillazine.org/Shortcut_to_a_specific_profile
Also don't forget to add -no-remote option to each of the links to enable starting such browsers in parallel. Sadly it's not mentioned behind the URL above.